The present invention pertains to a method and an apparatus for improved estimates of permeability of rock samples from capillary pressure measurements. In particular, the present invention provides an improved method for estimating the permeability of formations penetrated by borehole utilizing drill bit cuttings or sidewall core samples. In an evaluation of a formation penetrated by a borehole to determine whether it is capable of commercial production, two measurements are of importance; the formation porosity and formation permeability. The porosity of the formation is related to the amount of gas or oil contained in a formation while the measurement of permeability is an indication of the producibility of the trapped gas or oil. In particular, permeability of a reservoir of rock may be defined as the fluid conductivity or the ability of fluid to flow within its interconnected pore network. If there is no innerconnection between the pores, obviously the formation has zero permeability even though the formation porosity is high. Thus, there is no natural relationship between the permeability and the porosity of a formation.
U.S. Pat. No. 2,604,779 describes an apparatus and method for measuring the capillary pressure versus saturation of rock samples. In particular, the patent describes a system using a non-wetting liquid such as mercury for determining the capillary pressure versus saturation of the rock sample. The use of a nonwetting liquid provides a means for rapidly determining sufficient points along the saturation versus pressure curve to accurately define the curve. The inventor, W. R. Purcell, described in a paper in AIME Technical Publication No. 2544 of 1949 how measurements using mercury capillary pressures could be used to calculate the permeability of rock samples. In particular, the approach is to integrate the curve of mercury saturation versus the reciprocal of the capillary pressure squared. Obviously this approach involved detailed calculations and Purcell's approach was improved upon by various workers. One of the improvements was the observation that a log-log plot of capillary pressure versus saturation approximated a hyperbola and a mathematical expression for the capillary saturation data could be developed. Further, the hyperbolic functions could be related directly to permeability. While this was an improvement, it still required considerable calculations.
Also, the use of hyperbolic functions does not correlate with capillary pressure measurements made on small rock samples such as drill bit cuttings. The plateau of the curve in the low mercury saturation region is poorly defined and does not correlate with a hyperbola. Thus permeability estimates based on hyperbolic functions using data obtained from small rock samples results in overly optimistic estimates of the permeability. As a result, the formations which at one time appeared to be commercial are, in fact, not commercial formations.